Errors in calendar with German (Formal) translation

I use the Booking.com Official Search Box Search on several websites in German. The first impression is O.K., but when I open the calendar it is completely in English. The issue is only on websites with WordPress language “German (Formal)”. On all websites with normal “German” is the translation correctly. But I need on my websites “German (Formal)”. Please could you configure it so, that “German (Formal)” works with German language file too.

Thanks a lot!